How many users does Twitter have, really?

Through a pet-project of mine (foratweet.com) i accidentally stumble upon some interesting Twitter stats.

As it’s possible to extract your userid from the twitter-api, giving you a somewhat unique way to check how many users there are on Twitter. Given that they use a sequential userid-system, which many indications point toward.

So, how many does users twitter have?
Using the data from the API i found out the following statistics.